This invention relates to equipment for asphalt production plants. More specifically, this invention relates to a system to effectively heat and mix liquid asphalt in the liquid asphalt storage tank of an asphalt production plant.
Several techniques and numerous equipment arrangements for the preparation of asphaltic compositions for road paving, also referred by the trade as “hotmix” or “HMA”, are known from the prior art. Comparatively smaller quantities of such asphaltic compositions can be produced in batch plants. Alternatively, continuous production of asphalt compositions can be produced in a drum mixer asphalt plant. Typically, water laden virgin aggregates are dried and heated within a rotating, open ended drum mixer through radiant, convective and conductive heat transfer from a stream of hot gases produced by a burner flame. As the heated virgin aggregate flows through the drum mixer, it is combined with liquid asphalt and mineral binder to produce an asphaltic composition as the desired end-product. Optionally, prior to mixing the virgin aggregate and liquid asphalt, reclaimed or recycled asphalt pavement (RAP) may be added once it is crushed or ground to a suitable size. The RAP is typically mixed with the heated virgin aggregate in the drum mixer at a point prior to adding the liquid asphalt and mineral fines.
Both batch plants and continuous production, drum mixer asphalt plants include a storage tank for storing, heating and mixing liquid asphalt until this component is fed to a batcher or drum mixer in order to be incorporated into the final asphaltic composition product. Liquid asphalt must be continuously maintained above at least 300° F. in order to remain in a flowable, fluid state. Otherwise, the material will become extremely viscous, semi-solid or even solid if allowed to cool. Typical liquid asphalt tanks, therefore, include a large heating coil near the tank bottom through which is continuously circulated a heating oil to maintain the liquid asphalt at an elevated temperature in a flowable state. Such storage tanks also characteristically include a mechanical mixer with a rotatable shaft on which are mounted mixing paddles to circulate the heated liquid asphalt within the storage tank and to assist in keeping in suspension any additives that have been combined into the liquid asphalt.
Heretofore liquid asphalt storage tanks have presented several operational drawbacks. One of the chief problems is the tendency of the heating coil to “coke”, or build up with oxidized oil, both internally and externally which dramatically reduces the coil's ability to transfer heat to the liquid asphalt within the storage tank. Cleaning of the coil requires emptying the tank and then entering that hazardous environment through a manway with hazmat suits and steam pressure cleaning equipment to remove the coke buildup. If cleaning is unsuccessful, then the coil must be cut out and replaced. Either process is costly, as well as potentially dangerous to maintenance personnel.
The mechanical agitator equipment is likewise costly and difficult to maintain in the presence of liquid asphalt even under normal operating conditions. In the event of unexpected cooling of the liquid asphalt to a thick viscous or semi-solid condition, added strain on mixing paddles can damage drive motors and related moving parts.
A need remains in the asphalt industry for an improved liquid asphalt heating and mixing system to effectively address the foregoing problems relating to the operation and maintenance of a liquid asphalt storage tank. The primary objective of this invention is to meet this need.